The Question:

Why is "K" used to score strikes in baseball?

The Answer:

According to Total Baseball, The Ultimate Encyclopedia
of Baseball, Third Edition, sportswriter Henry Chadwick is
credited with inventing, in the 1850s, the baseball scoring system the
media and fans still use today. He was the first person to assign
letters for each play that could take place on the field and numbers
to every defensive player on the field. That way he could record the
result of every at bat easily and quickly. Chadwick needed to use the
letter "S" for sacrifice, so he decided a strikeout would be recorded
as the letter "K," which was the last letter in the word "struck"—a
common term for striking out.

Take note that when the letter "K" appears backwards it means
the player struck out "looking," or struck out without swinging at the
third strike.